Desensitizing inks are generally known to the one skilled in the art for the preparation of passivated regions or areas of chemically self-copying sets comprising at least two superimposed sheets, generally of paper, whose contacting faces are each coated with a layer. Usually, the top sheet comprises on its back surface a nucleophilic layer, and the underlying sheet has an electrophilic layer on its upper surface. A local pressure applied to the overlaying sheet produces a chromogenic reaction between the sheets so that the underlying sheet reproduces said local pressure. Multilayer sets or stacks are also known, based on the same principle.
Several examples of such inks have already been described; a most recent development made by the Applicants is disclosed in U.S. Pat. No. 5,035,743, patented Jul. 30, 1991, and in the corresponding European Patent No. EP-B1-0,333, 645. The whole disclosure of said U.S. and European patents is incorporated herein by reference. The disclosure of these patents explains in detail the chemistry of desensitizing and also the above mentioned four printing techniques. Thus, the principles of application and function of desensitizing inks will not be repeated in this paper.
A serious problem arising during the printing with desensitizing ink is the drying of the printings after the printing process. The drying time is of very high importance since, if the drying speed is too low, there is a set-off of the printed sheets which are laid down on stacks at the outlet of the printing press, or a set-off between following layers if the printed paper is wound on rolls.
It has already been tried to speed up the drying by curing the inks under the influence of energetic radiation. It should be explained here that desensitizing inks contain non-volatile liquids, and that the drying of the printings is mainly due to absorption of said liquids in the paper. To the contrary, a "drying" by radiation is effected by rapid polymerisation of polymerizable components of the ink, and this "drying" which is so called in the printing art, is in reality a radiation initiated chemical hardening or crosslinking. However, the substances and compositions which must be added to a conventional ink in order to render them curable by exposure to rays, make them inoperative since a strong diminution or a total absence of desensitizing power has been observed.
Thus, Japanese Patent Application No. 77/91,010 of Jul. 29, 1977, referenced in Chemical Abstracts Vol. 91, No. 25 of Dec. 17, 1979, reference 212,738a, discloses a desensitizing ink for pressure sensitive copying papers containing photosensitive binders and, as the desensitizing agent, a propylene glycol diglycidyl ether diacrylate. However, this compound is also polymerized under the conditions of ink polymerisation and does not show the excellent desensitizing power of Applicants' inks according to U.S. Pat. No. 5,035,743 as explained in that reference. Furthermore, that desensitizing agent is even free from nucleophilic hydroxyl groups.
U.S. Pat. No. 4,073,969 discloses a new method to desensitize the color developer coating. This method comprises covering the color developer with a cured film thereby to desensitize the color developer. In other words, the principle of that known method is to separate the color developer from the color former by a photocured film.